This invention relates to wall contacting tools useful in earth boring, especially by the rotary system, and more particularly to a fixed blade, rotating stabilizer whose wall contacting elements are in the form of replaceable wear pads.
In U.S. Pat. No. 3,680,647--Dixon and Crews, there is disclosed a wall contacting tool, specifically a fixed blade stabilizer, having replaceable wear pads of approximately sector shape cross section. The corner of each pad is received in a correlative V groove flute in the outer periphery of an arm of the tubular body of the stabilizer. The pad corner and flute apex angle is ninety degrees. The pad is secured to the body by a plurality of cap screws extending laterally through the pad into the two sides of the flute. Various prior United States patents referred to in the patent are numbered:
U.S. Pat. Nos. 3,454,308--Kennedy; PA1 2,084,421--Wright; PA1 2,973,996--Self; PA1 3,292,708--Mundt; PA1 2,088,770--Skinner; PA1 2,589,534--Buttolph.
The Kennedy, Wright and Self patents show various forms of wall contacting tools with wall contacting elements secured to the body by radially extending screws. Various and other forms of retention are shown in the Skinner, Mundt and Buttolph patents. Reference also may be made to U.S. Pat. Nos. 3,326,305--Garrett, 3,545,825--Hamilton, which are referred to in U.S. Pat. No. 3,818,999--Garrett.
In U.S. Pat. No. 3,818,999--Garrett, there is disclosed a stabilizer having wear pads of generally sector shape cross section, each disposed in a correlative 90.degree. angle V groove or flute in the stabilizer body. Each pad is held in place by dowel pins in the apex of the wear pad corner and body flute, the dowel pins preventing lateral movement of the pad. Each dowel pin has a head anchoring it to the body. Each dowel pin has a screw receiving socket and thus constitutes a nut, such nut receives a cap screw extending through a stepped bore in the pads to hold the pad to the body. Because there are a plurality of dowel pin-nuts for each wear pad, the dowel pin function of the dowel pin-nuts is less effective. This is by reason of the sloppy fit between the dowel pin-nuts and the bores of the pads and body in which they are received, as required to insure that the parts will fit together when made to reasonable tolerances. Because of the sloppy fit, shear loads are not confined to the dowel pin-nuts; the shear loads are transferred in large measure to the cap screws.
For the foregoing reason and perhaps others, the assembly can be loosened in use by excessive force, e.g. in directions parallel to the stabilizer axis (hereinafter referred to as axial force) caused, e.g. by the stabilizer, during a trip, passing through a tight place in the earth bore, causing the cap screws to be stretched beyond their elastic limit, or by excessive torque on the wear pads, due, e.g. to turning in a tight bore or while heavily pressed laterally against the side of the well bore, causing the cap screws to be stretched beyond their elastic limit, or by excessive vibration during rotation in the bore, causing the cap screws to turn relative to their nuts and work loose, or by normal vibration causing improperly tightened cap screws to come loose, or by obstructing material such as dirt or metal burrs at the mating surfaces of the assembly disappearing or reducing in size under the repeated stresses occurring when the stabilizer is in use, thus loosening the cap screws.
When the assembly is loose, the cap screws may fall out, the loose pad may break and fall off, the dowel pin-nuts may drop out, and the stabilizer body may be damaged by the movement of the loose pad relative to the body.
It is the object of the invention to overcome the aforementioned difficulties. It has heretofore been suggested that a retainer ring might be placed over the head of each cap screw, disposed in a groove in the wear pad. However, this does not prevent the cap screws from turning, for the wear pad can move with the cap screws away from the stabilizer body as the cap screws turn in the dowel pin-nuts.